Jupiter: Moon Festival

Jupiter: The Mini Solar System Grows

University of Hawaii astronomers have announced the discovery of 8 new satellites of Jupiter, bringing the total of known Jupiter satellites to 48.

Orbits of 48 Jovian Moons form a complex spirograph. The farthest 9 of them orbit a path nearly 300 times bigger than Jupiter itself, compared to the more modest Earth-Moon ratio of about 60:1 and a lunar orbit that more closely mimics the innermost Jovian satellite Io, at 380,000 km. In total the farthest new moon discovered orbits at a collosal distance more than 40 million kilometers away from Jupiter, but makes the orbital round trip once every one to two years. Credit: University Hawaii Institute for Astronomy

Jupiter has been referred to as a mini Solar System because of the thousands of small bodies it directly controls through its gravity. Since Jupiter is the largest planet in our Solar System it has influenced our neighborhood second only to the Sun. Until just the last 400 years, only the Earth had an observable natural satellite. But with Galileo’s first crude telescope, astronomers looked with initial success to the Giant Planet, what might have first seemed to them as just a bright star. Jupiter first uncloaked its four biggest moons, now called the Galilean system or inner Jovian satellites. Since Galileo, twelve times as many Jovian moons have been discovered.

The first 7 satellites were formally announced by the International Astronomical Union on Circular No. 8087 on March 4, 2003 while the eighth was announced on March 5. Two of the new satellites (S/2003 J1 and S/2003 J6) follow prograde orbits around Jupiter (ie. their orbital motion is in the same direction as Jupiter’s spin). The others have distant retrograde orbits like the majority of the known irregular satellites of Jupiter. However these orbits are still preliminary and may change as new observations are obtained.

How to Become a Moon-Hunter

Atop the remote 13,800 foot Mauna Kea volcano sits one of these mammoth 7 foot telescope lenses (88 inch, 2.2 m). Prospective moon-hunters wanting to use the Mauna Kea telescopes climb high enough to require a day to acclimatize at Hale Pohaku for one night before starting a new observing run. Compared to that first Galilean telescope that probed Jupiter, the tools are not only enormous but also precise. Even while the world’s largest observing domes of the nearby main Keck facility stand eight stories tall and weigh 300 tons, they operate with nanometer precision and move to instrument settings measured in units 10,000 times smaller than a human hair.

When first found from a series of 3 photographic panels or plates (typically taken about a half-hour apart), the visible objects that move against the relatively stationary star and galaxy background make the initial candidate list. But since asteroids between the Earth and Jupiter can also move against the galactic background, Sheppard and colleagues could only be sure of their discoveries by honing in on the characteristic slower speeds that might demark an orbitting object.

One Step Forward, Two Steps Back

Two images of S/2003 J1 showing the motion of the satellite relative to background stars and galaxies. Credit: UH

What is completely unknown about such irregular satellites is how they come to exist at all. When Jupiter was young, it is thought, many asteroids (or dynamical clusters) orbitted the Sun. As Jupiter condensed, its gravity began to bend the paths or even capture some of these stray asteroids. The best evidence for such a capture hypothesis is that many of these new satellites actually orbit in a direction opposite to the rotation of Jupiter, or otherwise follow what is known as retrograde orbits. Six of the new moons are retrograde.

But while the capture theory can explain the backwards orbits, that finding alone is only half the story of how actually to hold on to them once caught. The problem arises in slowing down the moon to a stable orbit. Following a large solar orbit requires lots of speed and energy, while going against the flow of Jupiter–if captured–is likely the only way to dissipate all that escape energy. At least for Jupiter in its present state, capture is almost impossible. As Jewitt noted: "The origin of the dissipation that lead to the capture of Jupiter’s irregular satellites is unknown. In fact, at the present time there is no plausible source of dissipation so that capturing satellites is presently almost impossible."

Only if Jupiter’s atmosphere extended well out into the capture zone, could friction start to slow down the moons and keep them from flying off or back into solar orbit. A much bloated Jupiter, and its atmospheric drag, could spawn such unusual tiny moons: captured into a backwards orbit, spanning vast relative distances but held just tightly enough to keep them as part of the 39 or more such moons. The best evidence for this theory appears to be the distinct families of moons that orbit at the same inclination to Jupiter’s equator as a kind of dynamic cluster that might have broken on capture. Indeed the Giant Planet has many characteristics of a mini-solar-system unto itself.

What’s Next

NASA hopes to launch a Europa Orbiter mission in 2008, with the primary goal of determining if there indeed is a global, subsurface ocean.Credit: NASA

Jupiter’s outer, irregular satellite system has long confounded predictions of what a moon should be. Fascinated particularly by the probable water-ice oceans on one of the six inner Jovian moons–Europa–moon-hunters and astrobiologists alike have begun employing some novel search strategies for even more exotic ones than Europa.

Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.

What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.

Digital camera (CCD) observations were obtained during Feb. 5-Mar. 4 at Mauna Kea with the 8.3-m Subaru telescope, the 3.6-m Canada-France-Hawaii Telescope, and the 2.2-m University of Hawaii telescope. The 8k CCD camera, was built at Hawaii’s Institute for Astronomy by Gerry Luppino and is one of the world’s largest astronomical cameras.